This article on the website of the US National Institute of Health:
… [sleep] triggers rhythmic waves of blood and cerebrospinal fluid (CSF) that appear to function much like a washing machine’s rinse cycle, which may help to clear the brain of toxic waste on a regular basis.
In an fMRI machine,
First, you see a wave of blood flow (red, yellow) that’s closely tied to an underlying slow-wave of electrical activity (not visible). As the blood recedes, CSF (blue) increases and then drops back again. Then, the cycle—lasting about 20 seconds—starts over again.
Fascinating that such a mechanical cycle exists. And that we can actually see it.
This brings me back to a fantastic longform National Geographic article in Aug 2018 about sleep, which had alluded to evidence about the same mechanism.
Also: “Why We Sleep” by Matthew Walker, one of the books I read last year.